【赛迪网－it技术报道】最近很长一段时间都在优化一个项目，这个项目存在许多问题，从数据架构到工作流

程，我一直在思考一些有关oracle性能优化的问题，我跟开发实施人员进行过多次交流，

发现存在许多交流障碍，许多问题实施人员觉得sql语句执行很快，并不存在性能问题，

我花了大量时间要他看执行计划，说明当数据积累到一定时间后，会执行越来越慢。

我举一个例子来说明问题：

select v2.*

from consultationrecorddoctor t1

right outer join

(select cr.*, crc.modifydate, crc.modifyuserid, crc.consultationtime,

crc.applyconsultationdeptid, crc.askconsultationdeptid,

crc.consultationdeptid, crc.casehistory_right,

crc.consultationorder_right, crc.consultationidea_right,

crc.otherhospital_right, crc.consultationdate,

crc.maindoctorid, c1.deptname_vchr as applydeptname,

c2.deptname_vchr as askdeptname,

c3.deptname_vchr as deptname,

f_getempnamebyno (crc.maindoctorid) as maindocname

from consultationrecord cr,

consultationrecordcontent crc,

t_bse_deptdesc c1,

t_bse_deptdesc c2,

t_bse_deptdesc c3,

(select cr.inpatientid, cr.inpatientdate, cr.opendate,

cr.createdate,

max (crc.modifydate) as maxmodifydate

from consultationrecord cr,

consultationrecordcontent crc

where (crc.applyconsultationdeptid = '0000208')

and cr.inpatientid = crc.inpatientid

and cr.inpatientdate = crc.inpatientdate

and cr.opendate = crc.opendate

and cr.status = 0

group by cr.inpatientid,

cr.inpatientdate,

cr.opendate,

cr.createdate) v1

where (crc.applyconsultationdeptid = '0000208')

and cr.inpatientid = crc.inpatientid

and cr.inpatientdate = crc.inpatientdate

and cr.opendate = crc.opendate

and cr.status = 0

and crc.applyconsultationdeptid = c1.deptid_chr

and crc.askconsultationdeptid = c2.deptid_chr

and crc.consultationdeptid = c3.deptid_chr

and cr.inpatientid = v1.inpatientid

and cr.inpatientdate = v1.inpatientdate

and cr.opendate = v1.opendate

and v1.maxmodifydate = crc.modifydate

and crc.maindoctorid is not null) v2

on t1.inpatientid = v2.inpatientid

and t1.inpatientdate = v2.inpatientdate

and t1.opendate = v2.opendate

and t1.modifydate = v2.modifydate

and t1.employeeflag = 1

where t1.employeeid is null

order by t1.employeeid desc, v2.askdeptname, v2.consultationdate;

查询使用了一个右连接，查询的表并没有包含t1表的字段，而且在实际的t1.employeeid中

根本不存在null的值，实际上是从一个大的结果集剔除一个大结果集的而获得一个小的结

果集，随着crc表数据量加大，会越来越慢！